Natural killer (NK) cells are an important component of the innate immune system and provide the first line of defense in the early stages of the immune response against viral infections, by production of cytokines and direct cytotoxicity. The killer immunoglobulin-like receptors (KIR) on NK cells regulate inhibition and activation of NK cell responses through recognition of HLA class I molecules on target cells, raising the possibility of an epistatic relationship between KIR and HLA in NK cell mediated immunity. We have shown that one activating KIR allele, KIR3DS1, in combination with HLA-B alleles that encode molecules having isoleucine at position 80 (HLA-B Bw4-80I) resulted in delayed progression to AIDS. The synergistic effect of these loci was most apparent on progression to CD4 T cell depletion, suggesting that a protective response of NK cells involving KIR3DS1 and its HLA class I ligands begins early after HIV-1 infection. High resolution genotypic analysis of families, as well as sequence analyses of KIR haplotypes indicate that the inhibitory KIR3DL1 and the activating KIR3DS1 segregate as alleles of one another. At least twenty additional alleles (all encoding inhibitory molecules) of the KIR3DL1/3DS1 gene have been identified. Some of the variants are not expressed, and others are expressed at either low or high levels. It is possible that the alleles might differ in their binding affinity for their HLA ligand, which may in turn influence AIDS progression. We hypothesize that this variation may play a role in AIDS pathogenesis. HLA-peptide tetrameric complexes (tetramers) of HLA-Bw4 complexed with HIV epitope peptides will be produced in order to determine binding affinity of KIR3DL1 alleles.The ligand for KIR3DS1 is not known, although our epidemiological data suggests that among HIV-1 infected individuals, it may be HLA-B molecules that have isoleucine at position 80. We intend to determine whether KIR3DS1 binds to HLA-B Bw4-80I molecules that contain HIV-1 peptide using an in vitro transfection system. If specific binding between this receptor-ligand pair occurs, then we will attempt to determine the HIV peptide contributing to this interaction. Thus far, four alleles have been cloned and preliminary analysis suggests that there are allelic differences in binding to HLA tetrameric complexes folded with HIV-1 peptides.